Re: [PATCH sched_ext/for-6.16] sched_ext: Add RCU protection to scx_root in DSQ iterator

[Tejun Heo]

On Wed, Apr 30, 2025 at 11:09:30AM +0200, Andrea Righi wrote:
> @@ -6907,28 +6907,39 @@ __bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
>  {
>  	struct bpf_iter_scx_dsq_kern *kit = (void *)it;
>  	struct scx_sched *sch;
> +	int ret = 0;
>  
>  	BUILD_BUG_ON(sizeof(struct bpf_iter_scx_dsq_kern) >
>  		     sizeof(struct bpf_iter_scx_dsq));
>  	BUILD_BUG_ON(__alignof__(struct bpf_iter_scx_dsq_kern) !=
>  		     __alignof__(struct bpf_iter_scx_dsq));
>  
> +	rcu_read_lock();
>  	sch = rcu_dereference(scx_root);

Can you instead try changing rcu_derefernece(scx_root) to
rcu_derefernece_check(scx_root, rcu_read_lock_bh_held())? The warning is
because this can be called both from rcu-locked and bh contexts and while
grabbing rcu_read_lock() once works, telling rcu that the dereference can be
in both contexts is more accurate here.

Thanks.

-- 
tejun